The 19th-century fascination in Europe with the architecture of the past begins with Greek temples and Gothic cathedrals, but soon extends to encompass a bewildering range of other historical styles - Egyptian, Byzantine, Romanesque, Venetian Gothic, Muslim Indian, and even, in a final convolution, the many Renaissance styles which are themselves a response to earlier periods.

This most self-confident of centuries takes what it likes from these many sources, mixes and matches them, develops and distorts them to create magnificent buildings. The effect is of its time, but the ingredients are not. Only one feature of 19th-century architecture is entirely new in the west - the use of cast iron.

Glass, iron and prefabrication: 1837-1851

The public first becomes aware of the glorious potential of cast-iron architecture in the 1840s, when extraordinary conservatories are erected at Chatsworth and in Kew Gardens. But the technology derives from factory construction in the 1790s.

With Boulton and Watt's steam machinery in operation, conventional factories using timber for joists and floors are prone to disastrous fires. The occasional use of cast iron for structural purposes goes back many centuries in China, for temple pagodas, but it is an innovation in Britain when William Strutt builds the first fireproof mill at Derby, in 1792-3, with floors on shallow brick arches supported on cast-iron pillars.

Strutt's mill still contains some massive wooden beams, but an entirely wood-free factory is constructed at Ditherington, near Shrewsbury, in 1796-7. Arched brick floors, on cast-iron beams and pillars, become the standard factory and warehouse interior of the 19th century.

The next and most glamorous stage in cast-iron architecture is linked above all with the name of Joseph Paxton. As superintendent of the duke of Devonshire's gardens at Chatsworth, he builds there in 1837-40 a great conservatory, shaped like a tent (277 feet long and 67 feet high) but consisting entirely of cast iron and glass.

In a ducal garden this building is not much visited, but it astonishes all who see it. Queen Victoria notes in her diary in 1842 that it is 'the most stupendous and extraordinary creation imaginable'. Two years later a similar building is commissioned from Richard Turner and Decimus Burton for the royal gardens at Kew. Since 1841 these gardens have been open to the public, so the beauty of the Palm House, completed in 1848, becomes more widely known than the Chatsworth conservatory.

But it is Paxton's building for the Great Exhibition of 1851, the astonishing Crystal Palace, which reveals to the millions the potential of the new architecture.

The Crystal Palace is gigantic compared to its predecessors in cast iron and glass. It is five times as long as the Palm House in Kew and nearly twice as high; or, put another way, it is longer than the palace of Versailles and higher than Westminster Abbey. But even more significant is the famous speed of its design (one week of detailed drawing, after a preliminary jotting by Paxton on a piece of blotting paper) and of its construction (six months).

The reason, and the reason for its lasting architectural significance, is that Paxton's building is the first thoroughgoing example of prefabricated architecture (a concept perfectly suited to cast iron, and pioneered seventy years earlier for the bridge at Coalbrookdale).

The statistics of the Crystal Palace are bewildering (3300 iron columns, 2150 iron girders, 250 miles of sash bar, 293,635 panes of glass), but the crucial detail is that these all conform to a basic 24-foot module. The manufacture of the pieces can be subcontracted to several foundries and glass factories; assembly on site is like putting together a giant's dolls' house. Hence the fact that this palace of glass is created, from scratch, in less than 200 days. As if to emphasize the point, it is dismantled in 1852 and moved to another site at Sydenham - where it stands until its contents catch fire in 1936.

The modular steel-frame tradition of late 20th-century architecture has in this building its most distinguished ancestor.